If doppler shift can be caused by gravity

In summary: So the question becomes, how do you determine the location of a high gravity place if it doesn't exist? (another trick question)
  • #1
hog
5
0
... In other words a photon is energy-mass equivalent so it is effected by gravity. If the photon comes from a place of very high gravity, and we see it shifted, does the emitter have to be moving? ( this is a trick question. I'm heading to Mach's Principle :-)

If high gravity can dopple shift a photon to zero energy, is that an event horizon? ( yeah yeah .. another trick question ).

If we look at the night sky, and it looks dark. Is that because there are no stars way out there, or are we seeing an event horizon that we call the night sky?

And if that were true, would the universe really have to be expanding? ( that's my real question )
 
Astronomy news on Phys.org
  • #2
The night sky is not dark to those with sensitive enough eyes and the capability to gather lots of light and take long, multiple exposures. In other words, telescopes. It is filled with EM radiation, but most of it is either too dim to see, or the wrong wavelength. The limit in brightness is not because the universe is expanding, although that does have an effect, it is because the universe is a finite age. This creates a temporal event horizon thanks to the finite speed of light, meaning that there is a point where we cannot see any further because there is nothing else to see! We are seeing the earliest point in time that we can see.
 
  • #3
hog said:
... In other words a photon is energy-mass equivalent so it is effected by gravity. If the photon comes from a place of very high gravity, and we see it shifted, does the emitter have to be moving? ( this is a trick question. I'm heading to Mach's Principle :-)
No it doesn't have to be moving. Gravitational redshift is a separate process from doppler shift.(and both are different from cosmological redshift)

If high gravity can dopple shift a photon to zero energy, is that an event horizon? ( yeah yeah .. another trick question ).
If we look at the night sky, and it looks dark. Is that because there are no stars way out there, or are we seeing an event horizon that we call the night sky?
You can't really redshift a photon to zero energy. There's always some energy left, although I suppose it could be infinitely small, and so, undetectable.

And if that were true, would the universe really have to be expanding? ( that's my real question )
"Aha!" you're probably thinking just now, "I've made the famous theory tremble in its foundations!". But hold your horses.
Your proposition, although unstated directly in your question, appears to be that in a static, spatially infinite universe, a photon traveling from any direction in the sky has got an infinitely massive universe pulling on it from behind, ergo, it should be infinitely redshifted.
However, consider that it's got an infitite amount of mass in front of it as well, so it should be at the same time infinitely blueshifted towards the direction of motion. Or, in other words, the effect would cancel itself.
There's quite a few more problems with it, but this one should be enough to dismiss the idea.
 
  • #4
hog said:
If the photon comes from a place of very high gravity, and we see it shifted, does the emitter have to be moving? ( this is a trick question. I'm heading to Mach's Principle :-)
The problem with this question is that it is based on a structure to the universe that is not observed. The universe is homogeneous. In other words, everywhere we look, it looks almost exactly the same. So the question is unanswerable because no such "place of very high gravity" exists.
 
  • #5
Bandersnatch said:
No it doesn't have to be moving. Gravitational redshift is a separate process from doppler shift.(and both are different from cosmological redshift)



"Aha!" you're probably thinking just now, "I've made the famous theory tremble in its foundations!". But hold your horses.
Your proposition, although unstated directly in your question, appears to be that in a static, spatially infinite universe, a photon traveling from any direction in the sky has got an infinitely massive universe pulling on it from behind, ergo, it should be infinitely redshifted.
However, consider that it's got an infitite amount of mass in front of it as well, so it should be at the same time infinitely blueshifted towards the direction of motion. Or, in other words, the effect would cancel itself.
There's quite a few more problems with it, but this one should be enough to dismiss the idea.

Yep. You guessed it. But if the universe is infinite, then the doppler shift is being caused by the gravitational lensing of the stars the photon passes on its way in .. and that would be a lot of stars. So the stars in front have no effect .. until the photon gets there.

I realize that doesn't explain the red shift of the photons "inside" the event horizon, but I'm working on it :-) Could it be that those photons are actually "lensed" from behind the local stars, and we just think they come from our "inside" the horizon stars ?
 
  • #6
russ_watters said:
The problem with this question is that it is based on a structure to the universe that is not observed. The universe is homogeneous. In other words, everywhere we look, it looks almost exactly the same. So the question is unanswerable because no such "place of very high gravity" exists.


Start from here and divide the universe into spherical shells of many light years thickness. The mass in each shell increases by R^3. It doesn't take very many shells to have the last one contain nearly infinite mass. And if the universe is infinite ... the mass is infinite in every direction. I think that almost has to generate an event horizon in reverse .. just like a Black Hole that prevents light from escaping. Infinite stars in every direction gives us Olber's Paradox. I don't think it has been properly explained away by local doppler shifting of photons in the known universe. I think that black sky is an event horizon caused by the Dark Matter that lies beyond it. ( Mach's Principle ).
 
  • #7
The mass is spread out. The volume of the universe increases just as fast as the mass does. So the "density" of gravity, which probably isn't the correct term, never gets larger. So no event horizon. This is unlike a black hole where the gravity increases as you get closer to the center, resulting in an event horizon forming.

Olbers Paradox is perfectly explained by the current model of the universe as finite in age and expanding.

Dark matter does not act like you imagine it does, nor does Mach's Principle.
 
  • #8
hog said:
Yep. You guessed it. But if the universe is infinite, then the doppler shift is being caused by the gravitational lensing of the stars the photon passes on its way in .. and that would be a lot of stars. So the stars in front have no effect .. until the photon gets there.

Uh, no. As light passes a source of gravity it is initially blueshifted on approach, and subsequently redshifted an equal amount as it leaves. There is no net shift either way.

I realize that doesn't explain the red shift of the photons "inside" the event horizon, but I'm working on it :-) Could it be that those photons are actually "lensed" from behind the local stars, and we just think they come from our "inside" the horizon stars ?

No. And personal theories are not allowed here on PF either as per the rules.
 
  • #9
hog said:
Start from here and divide the universe into spherical shells of many light years thickness. The mass in each shell increases by R^3. It doesn't take very many shells to have the last one contain nearly infinite mass. And if the universe is infinite ... the mass is infinite in every direction. I think that almost has to generate an event horizon in reverse .. just like a Black Hole that prevents light from escaping. Infinite stars in every direction gives us Olber's Paradox. I don't think it has been properly explained away by local doppler shifting of photons in the known universe. I think that black sky is an event horizon caused by the Dark Matter that lies beyond it. ( Mach's Principle ).
The universe is not currently thought to be infinite.
 

1. What is the Doppler shift?

The Doppler shift is a phenomenon in which the frequency of a wave appears to change as the source of the wave moves relative to the observer.

2. How is gravity related to the Doppler shift?

Gravity can cause the Doppler shift by affecting the motion of the source of the wave. This can result in a change in the observed frequency of the wave.

3. Can the Doppler shift be caused by gravity on Earth?

Yes, the Doppler shift can be caused by gravity on Earth. For example, the frequency of sound waves can be affected by the gravitational pull of a large object, such as a mountain or a moving vehicle.

4. Is the Doppler shift only caused by gravity?

No, the Doppler shift can also be caused by other factors, such as the relative motion of the source and observer, or the expansion of space in the case of light waves.

5. How is the Doppler shift used in scientific research?

The Doppler shift is used in a variety of scientific fields, including astronomy, meteorology, and medical imaging. It can provide valuable information about the motion and properties of objects, such as the speed and direction of a moving star or the flow of blood in the human body.

Similar threads

  • Astronomy and Astrophysics
Replies
3
Views
823
  • Astronomy and Astrophysics
Replies
13
Views
1K
  • Astronomy and Astrophysics
Replies
18
Views
3K
Replies
5
Views
879
  • Astronomy and Astrophysics
Replies
6
Views
1K
  • Astronomy and Astrophysics
Replies
7
Views
2K
  • Astronomy and Astrophysics
Replies
9
Views
2K
Replies
19
Views
1K
  • Astronomy and Astrophysics
Replies
2
Views
1K
  • Astronomy and Astrophysics
Replies
26
Views
2K
Back
Top